The present invention relates to a simple and easy method for the preparation of a printing plate for use in dry planography in the absence of dampening water.
In the prior art, various types of planographic printing plates for use in a dampening water-free printing process have been proposed. Those printing plates have drawbacks, such as, for example, poor performance as a printing plate and complicated process of manufacture. No satisfactory dry planographic printing plates are known hitherto.
As an example, a method was proposed in which a planographic printing plate is prepared by coating an aluminum base with multiple diazo type photosensitive layers formed of a diazo type photosensitive composition and an overlaying layer formed of dimethylpolysiloxane gum, then subjecting the thus coated aluminum base to exposure to light through a positive transparency overlaid to insolubilize the diazo layer in the exposed areas, removing the diazo layer in the unexposed areas in the developing process, and subsequently stripping the dimethylpolysiloxane layer off the unexposed areas (see Japanese Patent Publication No. 44-23042).
As a further example, a planographic printing plate was prepared by coating an aluminum base with a diazo layer, a layer of an adhesive and a layer of a silicone rubber successively, then subjecting the thus coated aluminum base to exposure to light through an overlaid negative transparency, allowing the photosensitive layer in the exposed areas to photodecompose for development, and stripping the silicone layer off the exposed areas (see U.S. Pat. No. 3,511,178).
In the above two methods wherein the layer of non-photosensitive silicone gum or rubber is present between the diazo photosensitive layer and the positive or negative transparency, any image patterns on the positive or negative transparency can not be reproduced with high fidelity on the plate and, since the stripping of the silicone layer is carried out by utilizing the change of the solubility of the photosensitive layer in the solvent, the edges of the image areas formed can not have clear-cut sharpness. Further, the plate making process of the dry planographic printing plate proposed in the prior art is disadvantaged by the complicated successive combination of the steps of providing multiple layers on the base plate, exposure to light and development.
As a solution of the above-described problems encountered in the prior art, the inventors of the present invention have recently proposed a method in which image areas are formed by exposing bare surface of the base plate after removal of the uncured silicone resin on the areas unexposed to light of a plate having a layer of a photosensitive or photocurable silicone resin on the surface (see U.S. Pat. No. 3,865,588).
The above method of making printing plates hitherto known are defective because of the complexity of the processes in which the removal of the silicone layer from the image areas is performed by the wet process using an organic solvent after exposure to light.
Apart from the above-described wet processes, dry processes are known for the preparation of a dry planographic printing plate. In most of the dry processes, the image areas of the plate are formed with a toner by the technique of electrophotography on the surface of the layer of a cured silicone covering the base plate. This dry process, however, is defective because of difficulty in obtaining sufficiently strong adhesion between the toner laid imagewise and the surface of the cured silicone, resulting in rendering the resultant printing plates poorly durable and unsatisfactorily printable.
For example, Japanese Patent Disclosure No. 48-19305 discloses a process in which a dry planographic printing plate is prepared by forming thermally fused toner images to adhere to the cured overcoating layer of a silicone on the layer of an electrostatic latent image-forming substance which has been applied on a base plate and dried. This method of plate making is not free from the above described problem, namely, poor adhesion of the toner to the silicone, since the silicone has been cured when the toner image is formed on it, resulting in the poor printing durability of the plate.
Various attempts have been made to improve adhesion between the toner and the surface of the silicone layer. However, no successful results have been obtained. For example, Japanese Patent Disclosure No. 49-21204 discloses a method in which the toner images formed on the electrophotographic layer are transferred on to the surface of the layer of a composition composed of a silicone gum and a resin, followed by bonding with thermal fusion to produce a planographic printing plate. The printing plate thus prepared is disadvantaged by poor ink-repellency because of a resin component included in the composition so as to improve adhesion between the toner and the silicone layer.
Further, Japanese Patent Disclosures 50-71405 and 50-71406 disclose a method directed to the improvement of adhesion between the toner and the layer of the silicone, wherein the toner is transferred and bonded by thermal fusion to the surface of the silicone layer formed by coating the base plate with an uncured or semi-cured silicone resin, followed by curing of the silicone to produce a finished planographic printing plate. The storability of the plate before curing is disadvantageously insufficient because of a thermosetting silicone resin used. In order to remove the problem of such storability, there was proposed the complicated means that a solution of a curing catalyst is applied on the layer of the uncured silicone resin after the toner images are formed on the surface of the silicone layer, and then the silicone layer is cured.
Japanese Patent Disclosure No. 51-16105 discloses a method for the preparation of a master for planographic printing plate which comprises forming on a base plate a coating layer of a silicone rubber composition containing two kinds of curing catalysts, then photosensitizing one of the catalysts, and thermally activating the other catalyst to cure the silicone rubber composition. This method is also defective in the complexity of formulating the curing catalysts, one being a photosensitive compound, such as an azide compound, and the other being a compound capable of being thermally activated, such as an organic peroxide, as well as the poor storability of the silicone rubber composition admixed with such catalyst system and the necessity of a heating step for sufficiently curing the silicone rubber composition.